1. Field of the Invention
This invention relates to an illuminating apparatus for a microscope, and particularly to an illuminating apparatus for a microscope in which a revolver is rotated to thereby enable an objective lens to be interchanged.
2. Related Background Art
In a prior-art illuminating apparatus for a microscope in which a revolver is rotated to thereby interchange an objective lens and change the magnification, there are provided a lamp as an illuminating light source and a power source for supplying electric power to the lamp, and the lamp is turned on to apply light to a sample on a stage.
Now, there are two types of illumination for a sample, i.e., illumination for bright field observation and illumination for dark field observation.
The bright field observation is microscope observation using a bright field illumination method which is a very popular illumination method, and basically, it is an observation method whereby light itself illuminating a specimen is caught by an observation optical system to enable the whole of the field of view to be seen brightly, and to achieve it, there are transmission illumination and reflection illumination.
Microscope observation using the transmission illumination is an observation method for observing chiefly living specimens or samples ready to transmit light therethrough, and according to this method, a specimen is illuminated with light from a light source and the light transmitted through the specimen is observed through an observation optical system including an objective lens. Accordingly, the illumination optical system and the observation optical system are constructed discretely from each other.
Microscope observation using the reflection illumination is an observation method for observing chiefly metallic specimens hard for the transmission of light therethrough, and according to this method, a specimen is illuminated with light from a light source through an objective lens and the light reflected by the specimen is observed through an observation optical system including the objective lens. Accordingly, the objective lens is a part of the constituents of both of the illumination optical system and the observation optical system.
FIG. 15 of the accompanying drawings shows the external appearance of a microscope for this reflection illumination.
Dark field observation also is microscope observation using dark field illumination which is generally known, and is for observing specimens which are low in contrast and difficult to observe by bright field observation because of the little difference in the unevenness of the specimens. Bright field observation is such that a specimen is illuminated in such a manner as to concentrate concentric circular light which surrounds the optical axis of an observation optical system toward the optical axis and this illuminating light from the oblique direction becomes scattered light in conformity with the unevenness of the specimen and the specimen is observed with this scattered light caught by the observation optical system. When scattered light is not created, the light does not reach the observation optical system and therefore, the whole field of view is dark and only that portion thereof in which scattered light looks shining. However, since this light is light in the dark, it looks shining, but it is very dark light as compared with the light for dark field illumination.
Accordingly, if the illuminating light directly enters this dark field of view, it will become very dazzling light.
A prior-art microscope in which bright field observation and dark field observation can be changed over is described in Japanese Laid-Open Patent Application No. 63-38911. This prior-art microscope, in which bright field observation and dark field observation are selectively effected, is provided with a changeover mechanism for selectively disposing a half mirror and a ring-shaped mirror above an objective lens.
FIG. 19 of the accompanying drawings is a cross-sectional view taken along the direction of the optical axis of a so-called bright and dark field observation objective lens 151 which is a dark field observation objective lens usable also for bright field observation. During dark field observation, a ring-shaped mirror, not shown, is disposed above the objective lens 151, and light reflected therefrom passes as incident light IL through a ring-shaped illumination system lens 157 provided around an objective lens unit 155, as indicated by dots-and-dash lines, is reflected on a conical reflecting surface 158 and reaches a specimen (sample) T. Scattered light from the specimen T spreads radially as indicated by dotted lines, and part of it passes through the objective lens unit 155 so as to be observed by an observer.
FIGS. 20A and 20B of the accompanying drawings are cross-sectional views showing an objective lens 151 for bright and dark fields pivotally movable with a revolver (not shown), together with the observation optical system 159 of a microscope body.
The state shown in FIG. 20B is a state in which dark field observation is being done. In the state shown in FIG. 20B, the dark field illuminating optical path 153 of the microscope body and the dark field illuminating optical path 154 of the objective lens are aligned with each other, whereby the incident light IL passes through the two optical paths 153 and 154 to the sample T. Part of relatively dark scattered light SL from the sample T passes through the objective lens unit 155 and the observation optical system 59 of the microscope body to the observer's eye.
The state shown in FIG. 20A is a state in which the optical axes of the observation optical system 159 and the objective lens 151 deviate relative to each other, that is, deviate from an observation optical path, in order to pivotally move a revolver (not shown) to select another objective lens. In such a state, the incident light IL passed through the dark field illumination optical path 153 of the microscope body passed not through the dark field illumination optical path 154 of the objective lens, but through the lens unit 55, and intense light condensed thereby is applied to a sample supporting plate, and the reflected light SL thereof which is several times as intense as ordinary light sometimes passes through the lens unit 155 and the observation optical system 159 to the observer's eye. This will hereinafter be referred to as undesired stray light. The observer who has so far been doing dark field observation is accustomed to seeing dark light and therefor feels dazzled when intense light suddenly enters his eye.
For example, when the number of fields of an eyepiece is 25, the range on the sample T illuminated through the lens 157 of an illumination system provided in the objective lens of FIG. 19 is the range of a circle of a diameter 5 mm for an objective lens of .times.5 times, and the range of a circle of a diameter 0.5 mm for an objective lens of .times.50 times. The rotational speed of the revolver when the objective lens is interchanged is generally constant and therefore, the lower is the magnification of the objective lens, the more of the illuminating light enters the observer's eye during the interchange of the objective lens.
The following expression (1) represents the relation between the quantity of light I for which the objective lens affects the observation system and the characteristic value of the objective lens. ##EQU1## where NA: the numerical aperture of the objective lens;
.beta.: the magnification of the objective lens.
As shown in expression (1), the quantity of light I is proportional to the square of the numerical aperture NA of the objective lens and is inversely proportional to the square of the magnification of the objective lens. In recent years, in objective lenses, the numerical aperture NA has been improved and thus, the quantity of light to the observation system increases, and many of objective lenses of low magnification have a high numerical aperture and therefore, the phenomenon of creating stray light becomes remarkable. This phenomenon may occur in all objective lenses for bright and dark fields.
Another problem in the prior art is that a microscope wherein a revolver is of an electrically driven type has a power source for driving the revolver discretely from a power source for an illuminating light source and this increases the costs of manufacture and a wide space becomes necessary for disposing respective power source circuits.